1. Field of the Invention
This invention relates to superconductive photoconductive oxide. The inventor performed experiments on optical properties, especially on the photoconductivity in response to high-speed pulses, of those oxides whose composition is outside that of regular oxide superconductors, and the inventor has found that oxides with a chemical formula Ba-Pb.sub.1-x -Bi.sub.x -O.sub.z, from 1 to 0.4 and z being 2.7 to 3, revealed an unexpected correlation between the superconductivity and the photoconductivity thereof.
The invention also relates to a method for of the above oxide. In the method, the ratio of the above element contents x and z is independently controlled for instance by changing x from 1 to 0.4, or by regulating the value of z through either quick cooling or slow cooling while keep x constant, e.g., at x=0.40. When the oxide has an x value of not smaller than 0.45 (x.gtoreq.0.40) or is quickly cooled, it becomes a semiconductor having photoconductivity. On the other hand, when the oxide has an x value of about 0.40 (x.perspectiveto.0.40) or is slowly cooled, it becomes a photoconductive oxide correlative with superconductivity. The oxide of the invention is expected to be very useful in the industrial field of superconductive optoelectronics.
2. Related Art Statement
There has been no publications at all on such superconductor which has inherent photoconductivity, except the inventor's disclosure of Y.sub.3-x -Ba.sub.x -Cu.sub.y -O.sub.z group and La.sub.2 -Cu.sub.1 -O.sub.z group substance in their academic papers and copending patent applications.
Conventional superconductors are metals or alloys thereof in the main. Recently, much attention has been paid to high-temperature oxide superconductors, such as superconductors of Y-Ba-Cu-O group, and considerable amounts of additives such as barium (Ba) and strontium (Sr) are used to modify or eventually raise the superconductive critical temperature (T.sub.c). Studies and measurements on the optical properties of the superconductive substances at and in the proximity of visible wavelengths have been limited to the study of reflection and scattering of light therefrom due to the metallic properties of such substance.
It has been believed that light is simply reflected from the surface of a superconductor and is not allowed to enter therein. Study of optical properties, except the phenomena of reflection and scattering, has been treated as a completely different field from that of superconductivity in academic institutions, domestic and abroad, and in international conferences.
The reason for it is in that, generally speaking, researchers have considered that superconductivity is incompatible with such physical properties as light absorption and photoconductivity and they have assumed that light irradiation in a wave number with the excess energy over the energy gap of the BCS theory will merely destroy the stability of superconductivity.
However, there is the certain clear correlation between them, as proven in the case of Y-Ba-Cu-O group substance and the like. If any substance having such photoconductivity which is related to superconductivity or any substance having both superconductive capability and photoconductive capability is produced, a number of new electronic and optoelectronic devices may be developed; for instance, a superconductive phototransistor, a "superconductive optical computer" as a combination of the "superconductive computer" based on the Josephson devices and the "optical computer" proposed in optoelectronics both currently studied, "superconductive optical fiber", and the like.
Therefore, an object of the present invention is to provide a photoconductive substance correlative with superconductivity which is an insulator in the dark and at a temperature below 14K and simultaneously has a photoconductivity Q(.lambda.,T) at a temperature (T) below 160K depending on the value of the above x upon excitation by light in a wavelength (.lambda.) range between 500 and 700 nm.
The superconductive photoconductive substance according to the invention has a general chemical formula of Ba-Pb.sub.1-x -Bi.sub.x -O.sub.z, x being 0.40 to 1 and z being 2.7 to 3 and the substance is a photoconductive oxide correlative with superconductivity which is an insulator in the dark and at a temperature below 14K and simultaneously has a photoconductivity Q(.lambda.,T) at a temperature (T) below 160K depending on the value of the above x upon excitation by light in a wavelength (.lambda.) range between 500 and 700 nm. With the method according to the invention, a mixture of starting materials is prepared for oxide with a general chemical formula of Ba-Pb.sub.1-x -Bi.sub.x O.sub.z, x being 0.40 to 1 and z being 2.7 ton 3 , and the mixture is heated at 750.degree.-850.degree. C. for 2-10 hours so as to cause solid phase reaction in the mixture. The heated materials are gradually cooled, and they are shaped under pressure and heated again at 500.degree.-850.degree. C. for 2-10 hours so as to effect secondary sintering thereon. The secondarily sintered materials are kept at 600.degree.-500.degree. C. for 2-3 hours, and then cooled either extremely quickly at a rate of 1500.degree.-900.degree. C./sec or slowly at a rate of 150.degree.-200.degree. C./hour, whereby the desired superconductive photoconductive oxide is produced.
The reason for limiting the composition of the oxide of the invention to the above-mentioned general chemical formula is that the oxide of the such composition reveals photoconductivity Q (.lambda., T) at a temperature (T) below 160K in an exciting wavelength (.lambda.) of 500 to 700 nm, that temperature being correlative with the onset temperature of superconductivity of a superconductor of the Ba-Pb-Bi-O system. The photoconductive substance is provided by a method comprising the steps of heating at about 750.degree.-850.degree. C. for 2-10 hours so as to cause a solid phase reaction among the starting materials thereof, cooling gradually, shaping under pressure, secondarily sintering at 500.degree.-850.degree. C. for 2-10 hours, keeping at 600.degree.-500.degree. C. for 2-3 hours, and then cooling either extremely quickly at a rate of 1500.degree.-900.degree. C./sec or slowly at a rate of 150.degree.-200.degree. C./hour. The photoconductive substance is an insulator or semiconductor in the dark and not superconductor in the composition range, even though the composition range is very close to that of superconductive substances of the Ba-Pb-Bi-O system, but the compositions herein do not include the composition range of a superconductor.
The inventor has found that, as to the values of x and z of the general chemical formula of Ba-Pb.sub.1-x -Bi.sub.x -O.sub.z of the oxide of the invention, if x is unity (x=1) or close to unity, the oxide becomes insulating and shows semiconductive properties as well as photoconductive properties for certain wavelengths of light even with higher on set temperatures. On the other hand, as x departs from unity and approaches to about 0.40, the superconductive properties of the substance becomes more apparent while the photoconductive properties of the oxide is maintained with various on set temperatures. It is noted, however, that if x.ltoreq.0.40 when z is in a range of 2.81 to 3, the oxide shows only superconductive properties and the photoconductive properties is lost, so that such oxide composition is excluded from the scope of the invention.